Should fatty acid proportions sum to 1?

What is it about?

Understanding animal diets provides essential insights into their ecology at scales ranging from individual animals to entire populations. Quantitative fatty acid signature analysis (QFASA) has become a popular method of estimating diets, especially for marine species. Although marine-origin lipids (fats) can be comprised of more than 70 fatty acids, some fatty acids are thought to be more informative with respect to diet than others. Consequently, investigators typically select a subset of all fatty acids thought to be primarily derived from diet, scale the proportions of the selected fatty acids so they sum to 1.0, and proceed with diet estimation. In this paper we show that this customary scaling can bias diet estimation in a way that has not previously been realized. We compare the current practice of scaling with two proposed alternatives, finding that the two alternatives avoid the scaling-induced bias and perform approximately equally well. We caution investigators against the traditional scaling of fatty acid signature proportions in QFASA.

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Why is it important?

This is the fourth in a series of papers in which we investigate various aspects of the performance of QFASA diet estimators and provide recommendations for changes to traditionally-used methods that improve the estimation of animal diet composition.